The Kepler team has pinpointed which of the 4,000 and more exoplanets could be similar to Earth.

In a new study published in the Astrophysical Journal, scientists outlined 216 Kepler exoplanets located within the "habitable zone," which is the area around a star where a planet's surface could hold liquid water. From the catalog, researchers listed 20 rocky planets that are the best second earth candidates.

"This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars," Stephen Kane, associate professor of physics and astronomy at San Francisco State University and lead author of the study, told SF State News.

"That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable."

According to Kane, the study also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of the planets outside of it, which means that life could exist elsewhere in the universe.

The researchers emphasized the importance of determining the boundaries of the habitable zone. Planets that are too close to its star will have a runaway greenhouse gas effect similar to Venus, while those that are too far have frozen waters.

After sorting out the planets according to their location in the habitable zone and their sizes, the researchers were able to narrow down to 20 planets that are most likely similar to Earth based on the most restrictive category: rocky surface and a conservative habitable zone.

"There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them," Kane said.

"This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth."

Kane, who is one of the world's leading "planet hunters," discovered the rocky planet Kepler-186f, which is included in the list. His discovery will be a huge contribution to two upcoming satellite missions - NASA's Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency's (ESA) Characterizing ExOPanet Satellite (CHEOPS).